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Pathology of Swine Influenza (H1N1 Flu)

Dr Sampurna Roy MD   







Swine influenza is an acute and highly contagious respiratory disease of swine caused by the orthomyxovirus.

Swine influenza is known to be caused by influenza A subtypes H1N1,  H1N2,  H3N1, H3N2,  and H2N3.

The most recent isolated influenza viruses from pigs have been H1N1 viruses.

Swine flu viruses cause high levels of illness and low death rates in pigs.

Swine influenza viruses may circulate among swine throughout the year.

The classical swine flu virus (an influenza type A H1N1 virus) was first isolated from a pig in 1930.

Like people, pigs can get influenza (flu), but swine flu viruses are not the same as human flu viruses.

Swine flu viruses do not normally infect humans.

However, sporadic human infections with swine flu have occurred in the past and have mainly affected people who had direct contact with pigs.

But the current swine flu outbreak is different.

The most worrying feature of the present outbreak is the swift human-to-human transmission of this virus.

It is caused by a new swine flu virus that is happening among people who haven't had any contact with pigs.

When the flu spreads from person-to-person, instead of from animals to humans, it can continue to mutate, making it harder to treat because people have no natural immunity.

The virus spreads when an infected person coughs or sneezes around another person.

People can become infected by touching something with the flu virus on it and then touching their mouth, nose or eyes.

Swine influenza viruses are not transmitted by food.

You cannot get swine influenza from eating pork or pork products. Eating properly handled and cooked pork and pork products is safe.

Cooking pork to an internal temperature of 160°F kills the swine flu virus as it does other bacteria and viruses.

There are no pathognomonic signs and symptoms of  of swine flu infections.

The symptoms of swine flu in people are expected to be similar to the symptoms of regular human seasonal influenza.

Fever  ;  Lethargy ;  Running nose ;  Cough ; Sore throat  ; Lack of appetite ; Nausea and vomiting and ; Diarrhoea in some cases.

Risk factors for severe disease: Extremes of age (children below 5 years and adults above 60 years ), lung disorders such as bronchial asthma, chronic obstructive pulmonary disease and pneumonia, pregnancy, obesity, patients on immunosuppressive therapy due to cancer or kidney disease and underlying medical illness like diabetes.

Pregnant women who were infected during the third trimester were at a higher risk for complications.

Main pathological findings are in the lungs: 

- Multifocal destruction and desquamation of the pseudocolumnar and columnar epithelia.

- Prominent submucosal edema and hyperemia.

- There may be thrombus formation at the bronchiolar level.

- The acute inflammation could be severe, as indicated by hemorrhagic tracheobronchitis and desquamative bronchiolitis with necrosis of the bronchiolar wall.

- Once necrosis occurs there is infiltration by polymorphs and mononuclear cells.

- The histological changes in influenza pneumonia included   

- Interstitial edema with inflammatory infiltrate,

- Alveolar proteinaceous exudation with membrane formation,

- Capillary thrombosis,

- Necrosis of the alveolar septa,

- Intra-alveolar hemorrhage,

- Desquamated pneumocytes with pyknotic nuclei into the alveolar spaces,  

- Diffuse alveolar damage with mainly mononuclear interstitial infiltration by lymphocytes and histiocytes.

- During the late stage, the following changes have been reported  1) organizing diffuse alveolar damage, fibrosis, 2)Type II pneumocyte hyperplasia, 3)epithelial regeneration, and 4) squamous metaplasia were found.

- These changes are in keeping with fibroproliferative phase of acute respiratory distress syndrome  (ARDS) and diffuse alveolar damage.

- Bacterial coinfections was found in a number of  autopsy cases.  The most common bacteria found  included Streptococcus pneumoniae, Streptococcus pyogenes, Staphylococcus aureus, community-associated methicillin-resistant S. aureus and Haemophilus influenzae.



Other pathological changes noted in autopsy examination:

Heart - Showed myocarditis myofibril degeneration and interstitial lymphocytic infiltrate.

Reticuloendothelial system (reactive hemophagocytosis).

Blood Vessel (vascular thrombosis) -May involve coronary and infrarenal aorta, and femoral and iliac veins with pulmonary thromboembolism.

Splenic infarction associated with thrombosed arteries.

Liver (hepatitis): Micro and macrovesicular steatosis.

Central Nervous system: Viral infection-associated encephalopathy.

Kidney: Renal failure.

One can help prevent the spread of germs that cause respiratory illnesses like influenza by:

- Covering your nose and mouth with a disposable tissue or handkerchief when you cough or sneeze.

- Wash your hands often with soap and water, especially after you cough or sneeze.  

- Try to avoid close contact with sick people.

- If you get sick with influenza, stay at home and limit contact with others to keep from infecting them.

- Avoid touching your eyes, nose or mouth.

- Consult your physician if you think you have any of the  symptoms.

To diagnose swine influenza A infection, a respiratory specimen would generally need to be collected within the first 4 to 5 days of illness (when an infected person is most likely to be shedding virus).

Some patients, especially children, may shed virus for 10 days or longer.

Identification as a swine flu influenza A virus requires sending the specimen for laboratory testing.


Past history of Swine Flu (H1N1) outbreaks:

The World Health Organisation (WHO) confirmed human cases of swine flu in Mexico, the United States, Canada and Spain.

Only Mexico has reported deaths from the new strain. The number of cases confirmed by health officials worldwide totaled 82,most  of them in the United States and Mexico, according to the WHO.

But hundreds more cases are under examination, and Mexico has said 149 deaths may have resulted.

The World Health Organization raised its global alert level on the spreading swine flu virus on Monday 27th April 2009, but stopped short of declaring a global emergency.

The United States advised Americans against most travel to Mexico and ordered stepped up border checks in neighboring states.

The European Union health commissioner advised Europeans to avoid nonessential travel both to Mexico and parts of the United States.

The suspected number of deaths rose to 149 in Mexico, the epicenter of the outbreak with nearly 2,000 people believed to be infected.

The number of U.S. cases doubled to 40, the result of further testing at a New York City school, although none was fatal. Other U.S. cases have been reported in Ohio, Kansas, Texas and California.

Worldwide there were 73 cases, including six in Canada, one in Spain and two in Scotland.

The WHO raised the alert level to Phase 4,meaning there is sustained human-to-human transmission of the virus causing outbreaks in at least one country.

Its alert system was revised after bird flu in Asia began to spread in 2004, and Monday was the first time it was raised above Phase 3.

Putting an alert at Phases 4 or 5 signals that the virus is becoming increasingly adept at spreading among humans. That move could lead governments to set trade, travel and other restrictions aimed at limiting the disease's spread.

The WHO's Phase 6 is the pandemic phase, characterized by outbreaks in at least at least two regions of the world.



History of Swine Flu (H1N1) outbreak in India:

According to the latest report as many as 1900 people have died due to swine flu. Total number of infected patients in India have gone upto 31,156. In West Bengal 23 patients have died due to the disease and the total number of infected patients is 323. (dated March 19th 2015)

Swine Flu death continues to rise in India. Total number of deaths has gone up to 774. According to the latest report nearly 13000 people have been infected. 

An eight year old child died in B C Roy Hospital, Kolkata. 11 new cases have been detected in last 24 hours. Now the total number of deaths in West Bengal is 3. (dated 21st February 2015).

Swine flu is rapidly spreading across India. Total number of cases tested positive for H1N1 influenza virus is more than 11,000. 40 more cases of deaths have been reported from various parts of the country. Total number of death due to swine flu has gone upto 703. (dated 19th February 2015 ). States mostly affected are Jammu and Kashmir, Gujarat, Rajasthan, Delhi and Telangana.

Since January only 2 deaths have been reported in West Bengal. According to health officials 6 new cases of H1N1 were reported in this state. Nobody is in critical condition. Total number of positive cases in the state is 42. There are no reports of death in last 24 hours. 

Both central and local state goverments are taking extensive measures to keep the situation under control.

In this post I have highlighted some important facts about Swine Influenza (H1N1 Flu)


Tuesday May 5, 2009:  India continues to be free from swine flu infection, while five passengers with suspected influenza A (H1N1) symptoms are quarantined.

With the World Health Organisation designating swine influenza outbreak as a public health emergency of international concern, India on Monday (27th April 2009) put on alert all international airports and ports for identifying persons with suspected infection who could be arriving from countries affected.

Those travelling to affected areas have also been advised to defer non-essential journeys. Instructions were issued to track down people who arrived in India from Mexico, the U.S., Canada, New Zealand, Spain, France and the United Kingdom.

Keeping in view the global scenario, the government is instituting a series of preventive action that includes surveillance at ports and international airports.  All passengers coming to India from the affected areas will be mandatorily screened for influenza symptoms while for others it will be voluntary.

The 21 airports in the country where screening is being done are: Ahmedabad, Amritsar, Bangalore, Calicut, Chennai, Cochin, Coimbatore, New Delhi, Varanasi, Goa, Hyderabad, Jaipur, Kolkata, Lucknow, Mangalore, Mumbai, Nagpur, Pune, Srinagar, Tiruchirapalli and Tiruvananthapuram.

Four labs - National Institute of Communicable Diseases (NICD) in Delhi. National Institute of Virology (Pune), National Institute of Cholera and Enteric Diseases (Kolkata) and Regional Medical Research Centre (Dibrugarh) - have been put on high alert to test human samples for H1N1 infection.

Doctors clarified that there is no risk of the flu from eating pork and pork products, including ham, salami and sausages. Otherwise, it is advised that pork be cooked at temperatures over 70 degrees - a level at which all viruses die. The medicine advised is Tamiflu and Relenza.]

Prevention by vaccination plays an important role. In addition to the prevention of pulmonary complications, influenza vaccination can prevent other medical hazards like myocardial infarction and stroke. 

The greatest setback in the 2009 influenza pandemic was that vaccine was not available in most countries.


Further reading:

Centers for Disease Control and Prevention (CDC) - Swine Flu

Pathologic Findings in Novel Influenza A (H1N1) Virus (“Swine Flu”) Infection -Contrasting Clinical Manifestations and Lung Pathology in Two Fatal Cases

Swine flu. Mexico's handling of A/H1N1 in comparative perpective  

A Study of the Swine Flu (H1N1) Epidemic Among Health

News and information on the 2009 human swine flu outbreak

H1N1 Swine Flu cases tagged on Google Maps

World Health Organization (WHO): Swine influenza

The Swine Flu Episode and the Fog of Epidemics by Richard Krause in CDC's Emerging Infectious Diseases Journal Vol. 12, No. 1 January 2006 published December 20, 2005

SWINE INFLUENZA by Carol G. Woodlief of College of Veterinary Medicine at North Carolina State University Overview, symptoms in pigs, treatment for pigs

Centers for Disease Control and Prevention - Key Facts about Swine Influenza (Swine Flu)

"Q & A: Key facts about swine influenza (swine flu) - Symptoms".

Swine workers and swine influenza virus infections.



Dr Sampurna Roy  MD

Consultant  Histopathologist (Kolkata - India)






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